In a conventional four-stroke cycle, internal combustion engine, a single piston in a cylinder completes a cycle of operation during two complete revolutions of a crankshaft. During an intake stroke, movement of the piston from top to bottom dead center creates a low pressure environment that draws air into the cylinder in preparation for the following compression stroke. In this manner, the flow of gas through the engine is aided by the pumping action of the piston during the intake stroke.
In a two-stroke cycle, opposed-piston engine, two oppositely-disposed pistons in a cylinder complete a cycle of operation during a single revolution of a crankshaft. The cycle includes a compression stroke followed by a power stroke, but it lacks a distinct intake stroke during which the cylinder is charged with fresh air by movement of a piston. Instead near the end of the power stroke, pressurized fresh air enters the cylinder through an intake port near one end of the cylinder and flows toward an exhaust port near an opposite end of the cylinder as exhaust exits. Thus, gas (charge air, exhaust, and mixtures thereof) flows through the cylinder and the engine in one direction, from intake port to exhaust port. The unidirectional movement of exhaust gas exiting through the exhaust port, followed by pressurized air entering through the intake port, is called “uniflow scavenging”. The scavenging process requires a continuous positive pressure differential from the intake ports to the exhaust ports of the engine in order to maintain the desired unidirectional flow of gas through the cylinders. Without this continuous positive pressure differential, combustion can falter and fail. At the same time, a high air mass density must be provided to the intake ports because of the short time that they are open. All of this requires pumping work in the engine, which is unassisted by a dedicated piston pumping stroke as in a four-stroke cycle engine.
The pumping work required to maintain the unidirectional flow of gas in an opposed-piston engine is done by an air handling system (also called a “gas exchange” system) which moves fresh air into and transports combustion gases (exhaust) out of the engine's cylinders. The air handling elements that do the pumping work may include one or more gas-turbine driven compressors (e.g., a turbocharger) and/or a mechanically-driven pump, such as a supercharger (also called a “blower”). In one example, a compressor is disposed in tandem with a supercharger in a two-stage pumping configuration. The pumping arrangement (single stage, multi-stage, or otherwise) drives the scavenging process, which is critical to ensuring effective combustion, increasing the engine's indicated thermal efficiency, and extending the lives of engine components such as pistons, rings, and cylinders. Manifestly, in a two-stroke cycle, opposed-piston engine, airflow is one of the most fundamental factors by which engine operation is controlled.
For effective control of airflow, information regarding the mass of incoming air (“mass airflow”) is vital to measurement of airflow conditions and to determination of precise and accurate control parameter values with which the air handling devices are actuated. Parametrically, mass airflow is often expressed in SI units, for example kg/s (kilograms per second). In many instances, measurement of air mass entering the air handling system of an opposed-piston engine is enabled by a mass airflow sensor positioned in a charge air channel of the air handling system. The dynamics of airflow pressure and temperature in the charge air channel subject such devices to extreme stress. Consequently, it is important to monitor the operation of a mass airflow sensor in an opposed-piston engine in order to ensure the integrity and accuracy of the mass airflow measurement which underpins the airflow-based control of the engine.
The above-referenced '612 application describes an on-board diagnostic system of an opposed-piston engine that monitors air handling system elements for proper functionality. Initial steps taken in preparation for on-board diagnosis of air handling elements include determining operational validity of air handling sensors, including a mass airflow sensor. A mass airflow sensor failure will stop an air handling diagnostic process since any further diagnostics would be invalid. Accordingly, it is desirable to enable the system to assess measurement functionality of the sensor.